Just looked at Ciconia’s Phase 1 Summary and I believe I just figured out the Cast for the massacre:
Miyao Mitake is Asumu/Piece Witch of the Piece, Meow Mitake is Battler, MIYAO is Kyrie, Miyao Mitake’s Father is Rudolf Ushiromiya, Jestress/Jayden is Beatrice/Featherine, Eua is Beatrice Castiglioni and Seshat/Chloe Ironside(who is implied in Higurashi Mei to be behind the Black Awakenings and is a Satoko) is the Ferry Captain.
That means that Miyao and Meow left their bodies due to the sheer trauma of the Ciconia setting followed by MIYAO creating the Ange of the Ciconia setting while Jestress in the role of Jayden(I.E. Shannon) started dating the George of the Ciconia setting only to get dissatisfied with her Jayden form and adopt the Featherine Augustus Aurora persona(I.E. Kanon).
That means that Featherine goes full Jestress at the end of Ciconia and initiated the start of the Rokkenjima Massacre only to be discovered by MIYAO followed by Lambdadelta killing one of her rivals by accident followed by Rosa’s Ciconia Counterpart getting deleted by MIYAO(a useless gesture if Rosa is a Brain in a Jar) who promptly deletes everyone he can get his paws on causing the Rokkenjima Massacre himself.
When he gets cornered by Lambdadelta(whom he fails to delete due to Lambda not being an AI) and made aware of his ending up being the AI that kills everyone(with Jestress/Featherine only existing because everyone believes she exists) he goes all supervillain and gets hit with a fatal wound that will either kill him or turn him into a Witch.
If Ange is an AI like Meow and Miyao then she and Meow will be merged with Featherine at the end and Ange will be shoved into Bernkastel to bring about a Logic Error turning Bernkastel into a Witch(Ange’s true purpose) while Meow will be shoved into Hanyuu with the possibility that exposure to Bernkastel makes Hanyuu to revive as Meow which causes Hanyuu to instinctively avoid Bernkastel.
Eva sending Ange to St. Lucia represents the Lambdadelta Virus Looping without Memories after Lambdadelta becomes a Witch possessing Rika & Satoko causing Higurashi When They Cry Gou/Sotsu/Meguri which inspires Bernkastel and Lambdadelta to go to St. Lucia themselves to play Rika & Satoko and do it more decently than the 2 strains of the Lambdadelta Virus did.
Ironically Seshat’s solution in Higurashi Mei(far from taking Bernkastel over to the Ciconia Setting like Aurora expects) is to strip Hanyuu of her Hanyuu identity giving her the Kazuho Kimiyoshi identity railroading her in a manner so that she has no choice but to become Meow Mitake even if she retains the Hanyuu form somewhat.
She might in that situation attempt to find clues to her other Companions from Ciconia such as Miyao Mitake(currently in Oblivion as Piece Witch of the Piece and thus waiting for the next Umineko Crossover with Higurashi Mei), Lambdadelta, Rirya/Cat Rika(probably unleashing a Cat Awakening to counter Seshat’s Black Awakening in retaliation for killing her in Ciconia Phase 1) and Koshka for instance.
Add Rosa, Jessica and Maria’s Ciconia Counterparts and Higurashi Mei gets very interesting… Jessica required multiple blows to kill her so her Ciconia Counterpart may infact be alive but having got the picture that the Ciconia Setting is off-limits due to MIYAO attacking her with the Delete Button and brutalizing her Avatar until her demise kicks her out of Ciconia’s Gameboard.
Also I suspect MIYAO to just be Eua playing the role of an AI just so that she can make sure that the story ends in Tragedy. She ultimately gets what she wants: 2 Fragments of herself(Meow and the Ange of Ciconia) to implant in others to create Witches resulting in Hanyuu(the failed knockoff she looks down on) and Bernkastel.